1. Field of the Invention
The present invention refers to a method of purifying flue gases from refuse incineration by cooling them and for recovering heat energy and to an arrangement for performing the method. In the purifying, acid components, such as hydrochloric acid and various sulphuric oxides as well as dust, heavy metals and other detrimental substances, are separated. The recovered heat energy may be utilized for example in a water system for heating.
2. Description of the Prior Art
It is known that flue gases from the combustion of conventional fuels, such as oil, coal, wood and peat, can be purified by being cooled to a temperature lying below the dew point. Some acid components, such as SO.sub.3 `and SO.sub.2, are separated at the time of condensation together with water. Heat can be recovered simultaneously and can either be transferred directly to a hot water system for example for heating rooms or alternatively the temperature of the heat energy is increased by means of a heat pump and can in such manner be transferred to a water system for heating. Cooling the flue gases generally is performed in one single stage from the initial temperature (150.degree.-250.degree. C.) to the final temperature (30.degree.-80.degree. C.).
Flue gases from refuse incineration include hydrochloric acid, and a very corrosive condensate is formed when cooling the flue gases to a temperature below the dew point. This condensate is much more aggressive than condensate from cooling flue gases of the fuels mentioned above. Also, the dew point is generally lower for flue gases from refuse as compared for example to oil. Another difference between refuse and other fuels is that refuse as a rule is a very cheap fuel, and therefore it is in most cases not profitable to utilize a heat pump for recovering heat energy from the flue gases.
For cooling different types of flue gases in the temperature range in which condensation can occur (generally lower than 150.degree. C.) three different types of material have generally been used, namely alloyed steels, glass and plastics or plastic-covered metallic material.
In cooling flue gases from refuse incineration it has turned out to be difficult for alloyed steels to withstand the corrosive attack if the concentration of the hydrochloric acid in the condensate and the temperature become too high. In order to obviate this it has been necessary to utilize cooling water having a temperature of 25.degree.-35.degree. C. when cooling is performed in one stage, with the temperature of the tube wall also being kept low, and not only hydrochloric acid but also a great amount of water condenses, with the concentration of hydrochloric acid becoming acceptably low. The disadvantage is that the heat energy of the cooling water lies at a low level (25.degree.-35.degree. C.), and if the energy is to be transferred for example to a district heating system this has to be done by means of a heat pump. The procedure of utilizing coolers of alloyed steel in for example two stages and withdrawing a cooling water having a higher temperature, for example 70.degree. C., cannot be applied in view of the danger of corrosion.
Glass is resistant to hydrochloric acid, and with a cooler of glass tubes it is possible to obtain cooling water having a temperature of for example 70.degree., wherein the heat energy may be transferred directly to a district heating system. A substantial number of types of plastic are also resistant to hydrochloric acid, but only a small number can withstand the high temperatures that the flue gases have in the beginning of the cooling process. Two relatively temperature resistant types of plastics are teflon and polyvinylidene fluoride, but the high cost of these types of plastics reduces the possibilities of utilizing them in the entire cooler structure when cooling flue gases from refuse incineration.
A disadvantage of glass and plastics is their poor heat transmission capacity as compared to alloyed steels. This becomes particularly noticeable in temperatures below the dew point in which the coefficient of thermal conductance is affected more by the heat conduction capacity of the material than at temperatures above the point at which condensation of moisture begins.
In cooling the flue gases in one stage down to below the dew point it is necessary either to make use of a corrosion resistant chimney or to heat the flue gases by 10.degree. C. to 20.degree. C. to prevent the chimney from corroding. Reheating the flue gases is the most common way of solving this problem, and if the heat energy has been produced by means of for example a heat pump this procedure becomes quite expensive.